The present invention is directed to a percussion blow added manually operable drilling tool, such as a hammerdrill, having a housing with a chuck and an electric motor within the housing for rotatably driving the chuck about its axis of rotation. A control device located within the housing for controlling the rpm of the chuck. A switch positioned on the housing for operating the electric motor and a percussion blow mechanism within the housing for periodically directing percussion blows to a bit secured in the chuck.
Percussion blow added or assisted manually operable drilling tools are used in the construction industry in installation operations and in the electrical trade and related trades. Due to the higher removal rate in hard materials being drilled, such as concrete, masonry and rock, hammerdrills have given particularly good results in professional use. Hammerdrills have a continuously driven chuck and a motor actuated percussive blow mechanism. In operation, percussion blow mechanisms automatically transmit blows in timed sequence to a bit clamped in the chuck for assisting in the removal of the material being drilled. At the same time, the bit inserted in the chuck is continuously rotated to produce a borehole for dowels in the receiving material. In known hammerdrills, with large material removal, for instance in a series of tools provided by Hilti AG, the percussion blow mechanism is an electropneumatic device which transmits axially directed blows to the tool bit during tool operation.
Each percussion blow drives the cutting edge of the bit tip into the material being worked and produces a small notch. Due to the continuous rotation of the bit, the material is sheared off. The angle swept by the cutting edge at the tip of the bit of the continuously rotating chuck between two blows is designated as the translation angle or angle of rotation. The translation angle is selected in such a way in known hammerdrills that a periodic repetition occurs only after several complete turns. In this way it is avoided that the cutting edge of the bit strikes against already existing notches in direct sequence and thus deepens these notches further. In case of periodic translation angles, such as 30.degree., 36.degree., 45.degree., 52.degree., 62.degree. and the like, the notches can be deepened to such an extent that the shearing action only removes an inadequate amount of material and the drilling output of the tool is greatly reduced. For this reason, such periodic translation angles are avoided by suitable design of the gear box in the hammerdrill.